/*红包雨*/
.red-envelope {
    position: fixed;
    top: 0;
    width: 100px;  /* 原来的两倍 */
    height: 140px; /* 原来的两倍 */
    background-image: url('red-envelope.png');
    background-size: cover;
    animation: fadeIn 1s, fall linear;
    opacity: 0;
}

@keyframes fadeIn {
    to { opacity: 1; }
}

@keyframes fall {
    to { transform: translateY(100vh); }
}